As a piezoelectric ceramic used for a piezoelectric ceramic element such as a piezoelectric ceramic filter, a piezoelectric ceramic composition including lead zirconate titanate (Pb(TixZr1-x)O3) or lead titanate (PbTiO3) as its main component is widely used. In order to realize a high performance of the piezoelectric ceramic whose typical example is a lead zirconate titanate, lead is an indispensable substance to be included.
However, it has been increasingly often pointed out in recent years that lead has harmful effects on a human body. Though a lead component included in lead zirconate titanate which is crystallized is relatively stable in comparison to an applied product including an amorphous lead, there is now a strong public demand that a piezoelectric ceramic in which lead is included by a least possible percentage become available also in terms of the environmental sustainability.
Further, lead oxides are evaporated in the manufacturing process of the piezoelectric ceramic including lead zirconate titanate or lead titanate as its main component. Therefore, there was another problem that variability of the piezoelectric properties was generated among the obtained products.
In order to solve the foregoing problems resulting from the inclusion of the lead component, a novel piezoelectric ceramic free of lead is demanded as a material used for piezoelectric resonators and radiators. Along with the trend, a piezoelectric ceramic of the alkali-niobate series has been attracting attention as a ceramic material showing such high piezoelectric properties without the inclusion of lead.
In the piezoelectric ceramic of the alkali-niobate series, sodium niobate (NaNbO3)(for example, see the Non-Patent Document No. 1) is an oxide whose crystalline structure is of the perovskite type (ABO3). However, the oxide cannot be used as the piezoelectric ceramic because it shows ferroelectricity properties only at a temperature lower than around −133° C. on its own, while failing to show piezoelectric properties in the range of −20 to 80° C. where the materials of piezoelectric resonators and radiators are generally used.
On the other hand, Some piezoelectric ceramics including potassium-sodium-lithium niobate (KxNayLizNbO3) as their main component thereof have a large electromechanical coupling factor and appear to be a promising candidate as the material for resonators such as piezoelectric ceramic filters and piezoelectric ceramic radiators (for example, see the Patent Documents No. 1 and No. 2).
Further, as another example of the piezoelectric ceramic not including lead, the piezoelectric ceramic of the bismuth-sodium titanate ((Bi0.5Na0.5)TiO3) series is known (for example, see Patent Documents No. 2 and No. 3).
[Non-Patent Document No. 1] Japanese Journal of Applied Physics, p. 322, vol. 31, 1992
[Non-Patent Document No. 2] Japanese Journal of Applied Physics, p. 2236, vol. 30, 1991
[Patent Document No. 1] Japanese Unexamined Patent Publication No. H11-228226
[Patent Document 2] Japanese Unexamined Patent Publication No. H11-228228
[Patent Document 3] Japanese Unexamined Patent Publication No. 2000-272963